The digital quadrature demodulation device is an indispensable part of an all-digital ultrasonic imaging system, and in particular an all-digital color Doppler ultrasonic imager.
FIG. 1 shows a typical ultrasonic imaging system (the emission part is not shown because of little relevance to this invention). A conventional imaging process is as follows. The probe emitting a pulse, each matrix element used for receiving receives the echoes, which, having been amplified and A/D converted, are then added in different time-delay amount to obtain the radio frequency (RF) data in the beam former. The RF data (i.e., x(t) hereinafter), divided into path I and Q, enters into the quadrature demodulation device, and then to B signal processing module, Color (or Colorflow) signal processing module or Doppler signal processing module depending on different imaging modes. Having been processed and converted by a digital scanning converter (DSC), they will be displayed on the screen in the form of comprehensible images. The master CPU is responsible for updating the parameters of each module.
FIG. 2 is a conventional quadrature demodulation device in the ultrasonic imaging system, in which I1, Q1 and I, Q have the same indications as those in the equations (1) and (2) hereinafter. The RF signal output by the beam former is synchronously assigned to two multipliers to be multiplied by the sine table value and cosine table value respectively. The sine table and cosine table values are obtainable from the sine table memory and cosine table memory. The output signals of the multipliers enter low-pass filters, which then pick up corresponding filter parameters from the filter parameter memory based on the depth represented by the input signals. The input signals are then filtered by the low-pass filter with selected filter parameters. Thus, the outputs are the quadrature demodulation results.
The ultrasonic wave in the medical equipment is characterized in that the center frequency varies with respect to the depth, so does the signal bandwidth. Corresponding to such characteristics, such a digital quadrature demodulation device is in need that supports the demodulation of signals of which the center frequency and bandwidth are variable. General digital ultrasonic equipments all realize the digital quadrature demodulation by using hardware (i.e., FPGA), and therefore in order for both the center frequency and the bandwidth to vary with respect to the depth (or time), large numbers of parameters need to be stored, which increase the overhead of the memory.
Assuming that the ultrasonic echo signal is expressed as:
x(t)=A(t)cos(ω(t)*t+Φ(t)), wherein A(t) is a low frequency signal, representing the amplitude variation of the echo with respect to different depth, ω(t) represents the frequency of echo, also variable with time, the quadrature demodulation process is realized in two steps:
(1)I1(t)=x(t)×cos(ω(t)×t)=A(t)/2(cos(Φ(t))+cos(2ω(t)×t+Φ(t)))Q1(t)=x(t)×sin(ω(t)×t)=−A(t)/2(sin(Φ(t))−sin(2ω(t)×t+Φ(t))).  (1)
(2) As seen from the above two equations, I1(t) and Q1(t) each consist of two parts of signals, that is, a low-frequency signal with the frequency close to 0 and a high-frequency signal with the frequency close to 2ω(t). The I1(t) and Q1(t) are respectively sent to low-pass filters to filter out high-frequency components. If the unit impulse response of the filter is represented as h(t), the following expressions are obtained:I(t)=I1(t)h(t)=A(t)/2×cos(Φ(t))Q(t)=Q1(t)h(t)=A(t)/2×sin(Φ(t)).  (2)
I and Q are the quadrature demodulation results. That is, after quadrature demodulating, the signals are divided into paths/and Q orthogonal to each other, wherein I and Q represent in-phase and quadrature respectively, and these two paths of signals mainly contain the low-frequency components of the original signals.
The modulus of I and Q, apparently A(t)/2, i.e., the amplitude information (envelope) of the original signal, is the base of type B imaging. With different calculations on the basis of I and Q, blood stream information concerning the diagnostic object will be obtained.
In an all-digital ultrasonic equipment, the quadrature demodulation is typically implemented by means of digital processing methods, wherein the equations employed to describe the above principle are expressed in digital form. As seen from above, the parameters concerning the quadrature demodulation include a sine table, a cosine table, and low-pass filter parameters. In an ordinary system, these parameters are all stored in a memory, and much more parameters are demanded because the demodulation frequency as well as the bandwidth should be variable.
Assuming that the AD sampling rate of the system is 40M; the desired detection depth is 30 cm; the sine table and cosine table length needed to store are approximately 16000 points; and the quantization precision of the quadrature demodulation, which generally should be higher, is 16 bit, then the memory space of the sine and cosine table should be 16000×2×16=512 Kbit. Assuming that the low-pass filter used for demodulating is 100 orders (higher orders may be desired in actual conditions); the bit width is 12 bit; and a set of filter parameters should be switched once every 64 sampling points (for meeting the bandwidth variable with respect to the depth), then a memory space of (16000/64)×50×12=150 Kbit is desirable. On one hand, such a design asks for additional memory chips in the system, and meanwhile increases the cost. On the other hand, the control thereof is rather complicated.